How to get my file back in 2.79 after saving with 2.80 series?
Restore your file from a backup. Check whether there is still a .blend1 or .blend2 next to your .blend, or pick a backup from another source.

If you didn't make backups before using a Blender 2.80 beta, you'll probably have to manually restore by trying to copy-paste bits and pieces between 2.80 and 2.79.

Note that the download page said Do not use in production nor save important files with it for a good reason.